The use of a parking brake to restrict the movement of a vehicle when it is parked is known. The parking brake may be, for example, a manual type in which an operating force is transmitted to a brake mechanism by the pulling of a brake cable by a operating lever, an electric type in which the rotational force of a motor is transmitted to the brake mechanism, or the like.
With the electric type parking brake, or EPB, to lock the EPB, the motor rotates in a locking direction (rotates in a forward direction), and the rotational force of the motor is transmitted to the brake mechanism, generating a braking force. In that state, the operation of the motor is stopped. To release the EPB, the motor rotates in a releasing direction (rotates in reverse), which cancels the braking force.
An EPB that performs this sort of locking and releasing control is disclosed in Japanese Patent Application Publication No. JP-A-2009-1151, for example. In the EPB that is disclosed in Japanese Patent Application Publication No. JP-A-2009-1151, basically, the termination of the releasing control is determined based on a motor revolution speed that is detected by a rotation sensor. In the event that the rotation sensor fails, the determination is made based on the electric current of the motor. Furthermore, when the releasing control is in effect, the braking force is regarded as having dropped to zero if the motor current becomes no greater than a specified value, at which point the motor is operated for a specified period of time, after which the releasing control is terminated. In other words, after the point when the braking force has dropped to zero, because a brake pad that is a friction-applying member has separated from a brake disc that is a friction-applied member, operating the motor for the specified period of time creates a specified clearance between the brake pad and the brake disc, after which the operation of the motor is terminated.
However, because the motor current varies according to changes in gear transmission efficiency and the like that are due to variations in temperature and the like, the time when the motor current becomes no greater than the specified value and the time when the braking force drops to zero are not necessarily the same time, as they are in the EPB that is disclosed in Patent Literature 1. Therefore, simply terminating the operation of the motor after the specified period of time has elapsed since the point when the motor current became no greater than the specified value makes it impossible to perform the releasing control with good precision, because the clearance that is provided between the brake pad and the brake disc is not constant.
According to Patent Literature 2, the releasing control can be performed with good precision by defining the point during the releasing control when a change (a differential value) in the motor current becomes no greater than a specified value as the point when the braking force becomes zero. In other words, the motor current varies for the reasons that are described above, but the change in the motor current does not vary for the reasons that are described above, so the time when the braking force drops to zero can be determined accurately based on the change in the motor current. Therefore, the clearance between the brake pad and the brake disc can be made constant, and the releasing control can be performed with good precision.